Electronics, Music and Technology

For Christmas, I asked my sister for a pair of Bluetooth headphones. I had wanted a pair for a while, but the biggest concern I had was quality, but hand in hand with that is price. I read many reviews about headsets in the $50-$80 range, and many of them complained of sound dropping, artifacting, tinny sound, empty bass. This was the highest I was willing to pay for BT headphones, and even at that, I wanted excellent quality for that price. Then I came across these:

These Nexxtech “Soaring” Bluetooth headphones were on sale for $19.99 at the time. I thought, “hey, these have the same kind of reviews as the $60 units. Since they are more affordable, and would be a great gift idea, I’ll try them out.” So, when Christmas came, these were the headphones I got.

I used them a few times, but it wasn’t until college resumed that they were getting more usage. I found they were fairly comfortable to wear (as long as you have hair that covers your ears), and that the sound quality was actually exceptional. Not as in, “I can tell it is Bluetooth but I don’t mind.” I mean, it sounded as close to wired headphones as it could get. My friends were equally impressed.

Unfortunately, clarity does not mean frequency response. The bass lacked punch, dubstep did not tickle my eardrums like other headphones. It suffered from Generic Driver Syndrome – standard, run of the mill, $10 headphone speakers.

Unfortunately, my sister was not aware that I help this motto close to heart, and she bought the extended 3 year warranty for me. A kind gesture, unfortunately lost in the face of experimentation and hardware modification.

Let’s begin – the first step was to remove the ear foams to reveal the screws.

After removing the screws, each side separates into two parts. The left side, along with the driver, contains the battery and USB charge port. The right ride contains the main PCB and microphone.

Left side – The USB port PCB has 3 distinct connections:
5V, Bat+, and Ground

Right side

On the main PCB, there are two sections of interest to us:

Audio output, where the speakers are connected

Power, where battery power enters, and for charging, 5V comes in from the left side USB port, is regulated, and sent to the battery.

Because one side of the headphones contains the battery and USB port, as well as a speaker, 5 wires are sent from the right to the left side: 3 power and 2 audio.

Now that the headphones were open, I began desoldering the connections to the main PCB, as well as to the USB port PCB. Note that I didn’t desolder the battery, just the wires going to the other side of the headphones.

This is what I ended up with. The old frame with drivers, the battery and USB port, and the main circuit board. Cool!

Choosing the recipient

The next question is “which pair of headphones shall I use?” Like most electronics packrats I have more than my share of headphones.

I am a big fan of earbuds, because they offer excellent noise cancelling, and channel bass quite directly to my ear drums. However, there is no way to mount this PCB to earbuds, unless I were to add a headphone jack to it, which in the end would completely defeat the purpose of going wireless.

I decided I would start my impedance matching – measuring the impedance of the stock drivers and finding a pair that matched, to ensure adequate volume, and so that I don’t overload the likely tiny amplifier inside. With my multimeter, I measured the impedance to be 32 Ohms, fairly standard for headphones.

First Candidate

It is difficult to choose something that you may be permanently removing its previous functionality, in an attempt to provide a different one, while not knowing whether it would be an improvement or not. I have two pairs of quality Sennheiser headphones – the HD 414 and the HD 540 reference. Although they have excellent sound quality, and frankly spend too much time on my shelves, I didn’t want to risk them in the name of science.

After gazing at them pensively, I turned my head to see the above JBL Bass Reference 410, found here. (On a side note, these headphones actually do have very good bass. Although they are not deserving of the $140 price tag they had upon release, at the $25 I paid it was well worth it, sound quality wise.)

Notice in the picture how similarly sized these are to the Nexxtech Bluetooth headphones. After measuring the impedance to also be 32 Ohms, I thought it was fate. But then…

First Rejection

I put them on and remembered the one thing I didn’t like about them. They are a little uncomfortable. They put a lot of pressure on my ears, and tend to slide on my head. After 30 minutes my ears would get sore. As well, they are adjustable in size in a way that would make it difficult to run the wire from one side to the other. As much as they sounded great and were the right size, it wouldn’t work.

Second Candidate

By this time (which was actually past 12am), I had already tested the sound quality over Bluetooth with the JBL headphones, and was pleased to find that bass did exist, and that it was simply a matter of cheap drivers. Good. I looked back at my Sennheisers, and thought, “even if I won’t damage these for the project, I can find out how much the higher impedance affects volume.”

The Sennheisers have 600 Ohm impedance, which is a lot higher than the stock 32 Ohms. This means that less current goes through the Sennheisers than the stock headphones at the same voltage level. That’s how I understand it, anyway, but the risk is that they may be much quieter.

I tested it anyway and found that, although the volume did need to be higher than usual, the highest volume setting was still more than I would ever listen, and sound quality was indeed incredible. My previous decision swayed.

We have a recipient!

My Sennheiser HD 540 reference headphones were the winner! I came to this decision around 1:30am because I remembered that the cables are removable from my Sennheiser headphones. That means I’m not doing any permanent modifications to the headphones themselves. At worst, I’m out a cable.

I measured the cables and cut them to length, shorter on the right side since that is were I’m mounting the Bluetooth receiver circuitry. Then, I soldered the wires to the PCB.

All the solder points on the PCB are actually pads, not through hole. Because of this, soldering was a bit difficult for the Sennheiser cables. As well, these cables were not standard copper like normal headphones (pictured above are copper wires from my experimenting.)

When I stripped the insulation, which was difficult because of how strong it was, I found that the wires were also like little steel cables. They were silvery in colour, and very stiff. They were almost solder-phobic, and it took a good deal of patience to not rage, now approaching 2am.

To tie it all together, once the headphones were soldered, and the battery as well, I took the two halves of the original headset and taped them together, and then taped this assembly to the headphones (electric tape is super). Unfortunately, my phone, and/or the headset, were suffering from a “failure to communicate”, and now at 2:30am, I went to bed before I got mad or worried that I damaged something somehow.

Final Touches (The Next Day…)

Morning came, and thankfully, all was well. They paired fine and sound came out. But they didn’t look great. They were lacking robustness, which is important to me. So, I set out to create a more durable solution.

Inspiration came from my mom, who told me to use Velcro. Indeed, that was a really good idea. Before I could do this, though, I needed to change the housing for the electronics. I thought about it, and after looking for some materials, I grabbed the lid from a yoghurt/margarine container. It was flexible, but would still make for a good backing. So, I cut out a piece to size, and screwed it to the back of the main PCB. To make the USB port fit, I had to file out an existing dome in the plastic to make a rectangle. After adhering some Velcro, this is what I had:

It looks a bit cleaner, despite not being sealed, and is now much easier to attach and remove than before. All the circuitry, battery and all, are sandwiched in there. The advantage of having the open edge, though, is that there is less strain on the stiff headphone wires, which should make it last longer.

So now, the moment of truth. Here is what it looks like now, completed:

Another successful project. I was indeed worried that I would take apart the Nexxtech headphones, start doing something with them, then either break them or lose motivation to finish the project. I checked the price of a new set beforehand, and discovered The Source raised it from $19.99 to $49.99! Outrageous.

A lot of thought went into this. In fact, I would say I spent a good 30 minutes total staring into space, thinking about how I would do this. The final stroke of brilliance was this morning, and I’m very glad that this project was accomplished!

If you meant that I could grab something like that, then shorten my headphone cables enough that I could mount it right to the headphones, that could work! But if you meant that I could just plug my headphones into that adapter, I’m back to where I started, since the point was to have completely wireless headphones.

@123
i think he did well with his solution. The Belkin receiver is lacking a battery and charging circuit and was not designed to drive headphones directly. He took a reasonably priced headphone with all the stuff already engineered for him and upgraded it by replacing the shabby drivers with very nice headphones. Very elegant solution 😉

Nice work here, but see if the first pair were uncomfortable but the original bluetooth was comfortable I would have just desoldered and replaced the drivers themselves and left everything else intact. A little dremel and superglue would handle things nicely, but then the nice pair would have been left sitting on a shelf and not been sitting used anyway I think. But since I pretty much only listen to audio books this would be a moot point for me, simple drivers pretty much handle my needs.

Had you read the entire (I’ll admit, lengthy) article, you would have noticed that I dismissed the first candidate for being uncomfortable, however chose the second candidate, the Sennheisers, because they were comfortable. A little dremel and superglue may work if the replacement drivers are of a similar size, however I did not have any such replacements on hand.